Digital Design Skills
Offices are using multiple different software and there is variation depending on the country and culture. I would advise learning basic concepts that do not vary that much between programs so you are able to adapt to different office cultures and office-specific ways of working.
First of all, make sure that you have the basic 2D CAD drawing skills in a commonly used CAD-software like Autocad.
Another thing many junior assistants work within offices is modeling and visualization. I would advise learning the modeling concepts in software like Rhino or SketchUp, and also to find out how models are imported and exported in and out from those programs as teamwork and workflows usually demand sharing and working with models across platforms.
In order to do image post-processing whether it is vector CAD or pixel renderings, you should have a basic photo editing and vector editing skills in programs such as Adobe Photoshop and Adobe Illustrator. You will be quite likely making lay-outs as well from time to time so the basics of layout program such as Adobe Indesign are handy. Not to mention Microsoft Office, as editing text documents and spreadsheets is still part of the office every day.
As your professional level increases and the expected skill-set accordingly: these days many building projects employ BIM (Building Information Management) -software and .ifc (Industry Foundation Classes) -format. In order to work in the production chain, architects are also required to output their projects accordingly. So it would make sense to learn to make BIM models with such software as Archicad and Revit.
The abovementioned skills should get you pretty far. Try to keep your skills up-to-date.
Tip! Regarding your desirability in the job market: it is never harmful to develop digital skills which are not that common and make you stand apart like Grasshopper or mastering a certain rendering software like Twinmotion!